bootstrap: emit hint if a config key is used in the wrong section

Author: lolbinarycat

src/bootstrap/src/core/config/config.rs

@@ -1853,13 +1853,7 @@ fn load_toml_config(
         } else {
             toml_path.clone()
         });
-        (
-            get_toml(&toml_path).unwrap_or_else(|e| {
-                eprintln!("ERROR: Failed to parse '{}': {e}", toml_path.display());
-                exit!(2);
-            }),
-            path,
-        )
+        (get_toml(&toml_path).unwrap_or_else(|e| bad_config(&toml_path, e)), path)
     } else {
         (TomlConfig::default(), None)
     }
@@ -1892,10 +1886,8 @@ fn postprocess_toml(
             .unwrap()
             .join(include_path);
 
-        let included_toml = get_toml(&include_path).unwrap_or_else(|e| {
-            eprintln!("ERROR: Failed to parse '{}': {e}", include_path.display());
-            exit!(2);
-        });
+        let included_toml =
+            get_toml(&include_path).unwrap_or_else(|e| bad_config(&include_path, e));
         toml.merge(
             Some(include_path),
             &mut Default::default(),
@@ -2398,3 +2390,54 @@ pub(crate) fn read_file_by_commit<'a>(
     git.arg("show").arg(format!("{commit}:{}", file.to_str().unwrap()));
     git.run_capture_stdout(dwn_ctx.exec_ctx).stdout()
 }
+
+fn bad_config(toml_path: &Path, e: toml::de::Error) -> ! {
+    eprintln!("ERROR: Failed to parse '{}': {e}", toml_path.display());
+    let e_s = e.to_string();
+    if e_s.contains("unknown field")
+        && let Some(field_name) = e_s.split("`").nth(1)
+        && let sections = find_correct_section_for_field(field_name)
+        && !sections.is_empty()
+    {
+        if sections.len() == 1 {
+            let section = sections[0];
+            if section == "<nesting error>" {
+                eprintln!("hint: section name `{field_name}` used as a key within a section");
+            } else if section == "<top level>" {
+                eprintln!("hint: try using `{field_name}` as a top level key");
+            } else {
+                eprintln!("hint: try moving `{field_name}` to the `{section}` section");
+            }
+        } else {
+            eprintln!(
+                "hint: try moving `{field_name}` to one of the following sections: {sections:?}"
+            );
+        }
+    }
+
+    exit!(2);
+}
+
+fn find_correct_section_for_field(field_name: &str) -> Vec<&'static str> {
+    let sections = ["<top level>", "build", "install", "llvm", "gcc", "rust", "dist"];
+    if sections.contains(&field_name) {
+        return vec!["<nesting error>"];
+    }
+    sections
+        .iter()
+        .enumerate()
+        .filter_map(|(i, section_name)| {
+            let dummy_config_str = if i == 0 {
+                format!("{field_name} = 0\n")
+            } else {
+                format!("{section_name}.{field_name} = 0\n")
+            };
+            let is_unknown_field = toml::from_str::<toml::Value>(&dummy_config_str)
+                .and_then(TomlConfig::deserialize)
+                .err()
+                .is_some_and(|e| e.to_string().contains("unknown field"));
+            if is_unknown_field { None } else { Some(section_name) }
+        })
+        .copied()
+        .collect()
+}